Electronic amplifiers for amplifying signals generated by analog sensors and/or transducers are known and have been extensively used in the automotive industry for decades. One particular class of transducers with which such amplifiers are often implemented are known as piezoelectric sensors, examples of which include pressure sensors, accelerometers and the like. Piezoelectric sensors are "self-generating" transducers in that they do not require externally supplied electrical power to generate output signals under dynamic mechanical loading conditions.
When dynamically loaded, piezoelectric sensors produce a high-impedance differential charge signal. In some applications, the ground-isolated differential output signal is amplified via a two-input signal amplifier, and in other applications one end of the differential charge signal is grounded (typically to the sensor housing) and a single-ended output is amplified via a single-input amplifier. In either case, electronic amplifiers for use with such piezoelectric sensors are operable to convert the high-impedance charge signal to a low-impedance voltage usable by signal processing circuitry such as a so-called engine control module (ECM), powertrain control module (PCM) or the like.
Generally, four basic amplifier configurations are used in the automotive and related industries for amplifying piezoelectric sensor signals:
a voltage amplifier using an operational amplifier, a charge amplifier using an operational amplifier, a current integrator using a number of operational amplifiers, and a unity-gain source follower amplifier using a field effect transistor. While each of the foregoing amplifier configurations are generally capable of appropriately conditioning the sensor output signals under some operating conditions, known embodiments of these amplifier configurations have certain drawbacks associated therewith and are accordingly incapable of satisfying demanding underhood requirements while maintaining desired operational characteristics as well as acceptable manufacturing goals (i.e., cost and ease of fabrication).
For example, one prior art sensor amplifying circuit 10 of the voltage amplifier type is illustrated in FIG. 1. Circuit 10 includes a piezoelectric sensor 12 having a single-ended input connected to an input VIN of a signal amplifier circuit 14. VIN is connected to a non-inverting input of a known operational amplifier 16, to one end of a capacitor C and to one end of a resistor R2, the opposite ends of which are connected to a REF output of circuit 14. The REF output is typically connected to ground potential in operation. The inverting input of amplifier 16 is connected to one end of another resistor R1 and to one end of a feedback resistor RF. The opposite end of R1 is connected to REF, and the opposite end of RF is connected to an output of amplifier 16 which provides the amplified sensor signal VOUT. Amplifier 16 requires connections to REF and to a power supply VCC for operation. While amplifier circuit 14 provides for satisfactory signal conditioning operation, operational amplifiers rated for underhood applications (i.e., -40.degree. C. to +150.degree. C.) are generally cost prohibitive.
An example of another prior art sensor amplifying circuit 20 of the charge amplifier type is illustrated in FIG. 2. Circuit 20 includes a piezoelectric sensor 12 having a sensor output connected to an input VIN of a signal amplifying circuit 22. VIN is connected to one end of a resistor R1, the opposite end of which is connected to an inverting input of an operational amplifier 24. The inverting input of amplifier 24 is also connected to one end of a feedback resistor RF and to one end of a capacitor C, the opposite ends of which are connected to an output of amplifier 24 which provides the amplified sensor signal VOUT. The non-inverting input of amplifier 24 is connected to one end of a resistor RA and to one end of another resistor RB. The opposite end of RA is connected to a power supply input VCC and the opposite end of RB is connected to a REF input which is typically connected to ground potential. The operational amplifier 24 must also be connected to VCC and REF for operation thereof.
Charge amplifiers of the type illustrated in FIG. 2 are widely used for amplifying signals produced by piezoelectric sensors and are commonly used in instrumentation applications employing pressure, force and/or acceleration sensors. The output voltage VOUT of amplifier circuit 22 is negatively proportional to the input charge and is determined solely by the feedback capacitor C. With the non-inverting input set at a DC reference voltage VREF, and the inverting input comprising a virtual ground node, the operational amplifier 24 drives the output in such a manner that the input voltages are equal. RF and C comprise a high-pass filter and determine the low frequency characteristics of the amplifier.
The amplifier circuit 22 has several practical drawbacks associated therewith. For example, as with amplifier circuit 14 of FIG. 1, an operational amplifier 24 rated for underhood applications is typically cost prohibitive. Moreover, circuit 22 has a long power-up delay (up to 10 seconds) due to the large component values often required for RF and C. The circuit configuration illustrated in FIG. 2 can be enhanced to address the foregoing deficiencies but doing so undesirably adds further cost to sensor circuit 20.
An example of another prior art sensor amplifying circuit 30 of the current integrator type is illustrated in FIG. 3. Circuit 30 includes a piezoelectric sensor 12 having a sensor output connected to an input VIN of a signal amplifying circuit 32. VIN is connected to an inverting input of a first operational amplifier circuit 34 and to one end of a first feedback resistor RF1. The opposite end of RF1 is connected to an output V1 of amplifier 34 and to one end of a capacitor C1. The opposite end of C1 is connected to one end of a resistor R1, the opposite end of which is connected to an inverting input of a second operational amplifier 36, one end of a second feedback resistor RF2 and one end of a capacitor C2. The opposite ends of RF2 and C2 are connected to an output of amplifier 36 which provides the amplified sensor signal VOUT. The non-inverting inputs of amplifiers 34 and 36 are connected to a REF input which is typically connected to ground potential. As with the amplifier circuits of FIGS. 1 and 2, amplifiers 34 and 36 include connections to an external power supply VCC and to REF.
Amplifier 34 comprises a current to voltage converter which provides an output proportional to the change in sensor output. C1 blocks the DC component of V1 and amplifier 36 comprises a conventional voltage integrator and integrates the AC component of V1 to produce a signal proportional to the mechanical force acting on sensor 12. As with the amplifier circuits of FIGS. 1 and 2, the cost of operational amplifiers 34 and 36, if rated for underhood applications, is cost prohibitive. Moreover, the size and cost of capacitor C1 is excessive and accordingly impractical for use integral with the sensor 12.
An example of another prior art sensor amplifying circuit 40 of the unity-gain source follower FET type is illustrated in FIG. 4. Circuit 40 includes a piezoelectric sensor 12 having a sensor output connected to an input VIN of a signal amplifying circuit 42. VIN is connected to a gate of a p-channel enhancement mode metal oxide semiconductor field effect transistor (MOSFET) M1, to one end of a resistor R3 and to one end of a capacitor CR. The opposite end of R3 is connected to one end of a resistor R1 and to one end of a resistor R2. The opposite end of R1 is connected to the source of M1 and the opposite end of R2 is connected to a reference input REF that is typically connected to ground potential. The drain of M1 is connected to one end of a drain resistor RD, and the opposite end of RD, as well as the opposite end of CR, is connected to REF. The drain of M1 is also connected to the base of a NPN bipolar transistor Q1 having a collector connected to the source of M1 and an emitter connected to REF. A current source 44 receives electrical power from an external source VCC and has an output supplying a source current I.sub.S to the source of M1. The common connection of current source I.sub.S, source of M1, resistor R1 and collector of Q1 defines the output of amplifier circuit 42 which provides the amplified sensor signal VOUT.
The configuration of MOSFET M1 in amplifier circuit 42 is well suited for amplifying high-impedance signals from piezoelectric sensors since its input impedance is high, its output impedance very low and the amplifier gain is near unity. The drain resistor RD and NPN transistor Q1 are used to bias M1 for improved linearity and dynamic range. The feedback path established by resistors R1, R2 and R3 is used to properly bias the gate of M1 and to properly bias the output of the amplifier circuit 42.
While some of the characteristics of the amplifier circuit 42 illustrated in FIG. 4 are attractive for use integral with a piezoelectric sensor (i.e., small size, avoidance of operational amplifiers, etc.), circuit 42 has several drawbacks associated therewith. For example, most circuits of this type require a supply voltage (VCC) in the range of 18-30 volts which is typically not readily available in automotive environments. In addition, biasing of the output VOUT limits the dynamic range of the circuit. Moreover, the output signal VOUT is AC coupled, whereas a DC coupled output signal is preferred for diagnostic purposes. Further, most circuits of this type require excessive current consumption (e.g., &gt;20 mA). Further still, the circuit 42 exhibits slow power-up due to the feedback network used for biasing of the gate of M1.
What is therefore needed is a signal amplifying circuit suitable for use with a piezoelectric sensor that is robust enough to withstand harsh underhood environments while also satisfying the goals of maximizing amplifier performance and minimizing size and amplifier cost. Such an amplifier circuit is preferably packaged integral with the sensor itself while also overcoming the shortcomings associated with the various prior art amplifier circuits just described.